A three-dimensional displacement measuring system has been known as a displacement measuring device which is connected to a displacement measuring object which is relatively displaced and measures, for example, a relative three-dimensional displacement (refer to Patent Literature 1). The three-dimensional displacement measuring system accurately measures a behavior having six degrees of freedom of the displacement measuring object and realizes a system which is cheaper and smaller than a non-contact type system. Positioning of the three-dimensional displacement measuring system to the displacement measuring object is easy.